Video game nation: Why so many play

Fries sits in his parents' living room in Virginia, wearing a faded green Peace&Love T-shirt, controller in hand. He's got Skyrim up on the big screen. His father and a friend, Tom Harvey, watch.

No dragons this time. His character has a more limited quest: making his way across frozen tundra toward a town. As he travels, Fries makes choices for him.

Fight or retreat? Enter a cavern or choose another way? Walk slowly or run ahead? It's what Fries has done playing video games for more than a decade.

"The average young person racks up 10,000 hours of gaming by the age of 21," says Ms. McGonigal. Ten thousand hours. It's a number made famous recently by Malcolm Gladwell's book "Outliers." In it, he offers the 10,000-hour-rule, based on research by a Swedish psychologist who argues that it might take that much time to become really good at something.

Pianists do it. Why not gamers? Forty-hour-a-week gamers might seem scary. Fries and Harvey are more typical. They met when Harvey managed one of the 6,500 stores in the GameStop chain, now the largest American retail outlet for video games. When Fries turned 16, Harvey hired him as his assistant.

Games don't totally dominate their lives. Fries keeps up with schoolwork. Harvey now manages a clothing store. They are articulate, funny, and take showers. But they've both put in their 10,000 hours – including entire days on weekends.

And Harvey illustrates something else. The average American gamer is about 37 and has played for 12 years. Harvey is 29. He's played for 13 years. This isn't something kids outgrow.